The Third Generation Partnership Project (3GPP) is developing a Long Term Evolution (LTE) system using a physical layer based on globally applicable evolved universal terrestrial radio access (E-UTRA). A mobile terminal or station (MS), also referred to as User Equipment (UE), may use a measurement based on a cell-specific reference signal as a metric to determine if a radio link with the base station is in synchronization or out of synchronization by determining whether reliable transmission of a physical downlink control channel (PDCCH) codeword with specific downlink message formats can be supported over the link.
In Release-8/9/10 of 3GPP, the UE Layer 1, such as or the physical layer, monitors the downlink radio link quality of the serving cell, such as from a base station, for the purpose of indicating radio problem detection status or radio link quality to higher layers. The radio problem detection may be based on cell-specific reference signals. In non-Discontinuous Reception (non-DRX) mode operations, such as when the UE is not in sleep mode, the UE in every radio frame checks the quality, measured over a time period, against thresholds (also known Qout and Qin) defined implicitly by relevant requirements for detecting out-of-sync (OOS) and in-sync (IS) conditions, where the term “sync” is synchronization. For every radio frame, the UE indicates radio problem detection to higher layers when the quality is worse than the threshold Qout and continues monitoring until either (a) the quality gets better than the threshold Qin, or (b) a radio link failure (RLF) is declared (after a certain timer expires) and the radio link recovery procedure is initiated. Typically, a UE experiencing radio link quality issues with one serving cell (or enhanced Node B (eNB) or enhanced Base Station) may be handed over to another serving cell (based on measurements provided by the UE to the eNB or network-aided measurements). However, for cases, e.g., where a UE in a connected state to a serving cell, but the UE suddenly experiences severe sustained quality degradation and cannot receive any messages from the serving cell, radio link failure occurs and radio link recovery procedure is considered useful.
Typically the requirements are defined based on whether or not a reference PDCCH Block Error Rate (BLER) is achieved for a particular downlink control channel configuration. For example, for the Rel-8/9/10 LTE, the OOS is reported to the higher layers from the lower layers if a hypothetical or reference PDCCH BLER becomes greater than 10% assuming the transmission of a Downlink Control Information (DCI) Format 1A at an eight CCE aggregation level, which, for example, corresponds to a small payload size Downlink (DL) grant (used for scheduling data or broadcast control transmissions) with the highest code protection (due to using eight CCEs where eight is the maximum that can be assigned for a DCI). An IS condition is reported if the hypothetical or reference PDCCH BLER drops below 2% assuming the transmission of a DCI Format 1C (with a certain different payload size) at a 4 CCE aggregation level, such as the downlink control message associated with (for scheduling) the transmission of a paging message or system information message (which may be typically broadcast information). In Rel-10 enhanced Inter Cell Interference Coordination (eICIC), the base station (or eNB) can further configure the UE to monitor the radio link quality in only a subset of subframes. Based on the OOS and IS events, if it is determined that the radio link quality is poor, Radio Link Failure (RLF) may be declared.
In Rel-11, an enhanced control channel, such as an enhanced PDCCH (ePDCCH), may be specified for an ability to support increased control channel capacity, an ability to support frequency-domain ICIC, an ability to achieve improved spatial reuse of control channel resource, an ability to support beamforming and/or diversity, and/or an ability to operate on the new carrier type and in Multicast/Broadcast over a Single Frequency Network (MBSFN) subframes.
If an enhanced control channel is specified, then there is a problem with adapting Radio Link Monitoring (RLM). In particular, there is a need for performing RLM at a UE on a Uu (eNB-UE) link when the eNB uses ePDCCH as a control channel to communicate with the UE.